The use of two-component adhesives based on polyhydroxyl compounds and polyisocyanates is known from GB-PS No. 1,408,510 and DE-A No. 2,719,720. These systems always require a time consuming process of mixing and above all the optimum proportions of hydroxyl groups to isocyanate groups must be accurately observed, which is not always possible in practice. It is for this reason that there is an increasing demand for one-component systems which can simply be applied without previous mixing and dosing. The main one-component systems which have so far been developed are the moisture hardening systems, which have the disadvantage that they only harden very slowly owing to the slow diffusion of water, especially in thick bonding joints.
One-component systems which can also be relied upon to harden in thick joints are the heat curable systems. One example of such a system disclosed in DE-A No. 3,403,499 consists of dispersions of polyisocyanates in polyhydroxyl and/or amino compounds in which the polyisocyanates have been inactivated by certain operations.
The use of high melting hydroxyl compounds in systems containing isocyanate groups is described in U.S. Pat. No. 2,908,657. In these systems, starch was used as the polyhydroxyl compound. The possibility of using such systems as adhesives was described in U.S. Pat. No. 3,488,302 in which pentaerythritol in particular is used as hydroxyl component in a quantity of from 2 to 100 hydroxyl groups to one isocyanate group. Very flexible systems are described but these only harden slowly. EP-A No. 85,243 also describes systems containing isocyanate groups in which pentaerythritol is used as curing agent under the condition that the ratio of hydroxyl groups to isocyanate groups is less than two and preferably about 1. These systems have the disadvantage that the curing temperature is still too high but attempts to accelerate such systems by means of the conventional catalysts of isocyanate chemistry resulting in a loss of stability in storage which again limits the possibility of using these systems.
It has now surprisingly been found that storage stable systems which harden at temperatures below 120.degree. C. are obtained when metal salts of fatty acids containing more than 11 carbon atoms and having melting points above 100.degree. C. are used as catalysts. The catalytic activity of zinc stearate on the reaction of hydroxyl groups and isocyanate groups has been described in U.S. Pat. No. 4,119,594 but only for elastomers with limited pot life produced by the RIM system.
The positive effect of metal salts on the adhesive properties of the systems according to the invention is surprising since salts such as zinc stearate have always been used as mould release agents for plastics parts to facilitate their removal from metal moulds as well as in the RIM process (see U.S. Pat. No. 4,585,803) and it was therefore not to be expected that these additives would improve the bond strength of adhesive systems, especially on metals such as steel.